This invention relates broadly to semiconductor devices, and to methods for making such devices and particularly to display devices such as field emission displays.
Because of their exceedingly small size, it is often very difficult to align one feature in a semiconductor device with another. The problem arises from the fact that once one feature is in place on a semiconductor layer it is very hard to subsequently align another feature with that feature. For example, it may be difficult to align an etching mask with the first feature because there is no physical way to guide the mask into alignment with the first feature.
One way of overcoming these problems is to self-align two features to one another. For example, one self-alignment technique is to use the gate electrode as a mask for the subsequent source/drain ion implantation. Because the gate electrode acts as a mask for the implantation, the source and drain regions become aligned to the gate electrode.
Self-alignment may be advantageous since it may allow devices to be made smaller because some misalignment tolerances need not be included in the design. These misalignment tolerances may require the device to be larger to compensate for the misalignment between features.
Moreover, in many applications the misalignment influences the operating effectiveness of the device. Thus, it is highly desirable to coordinate features such that subsequent features are aligned with previous ones.
Another example of a self-alignment technique applied to semiconductor devices arises in the field of field emission displays. Field emission displays use electron emission from an emitter to illuminate a screen which displays a corresponding image for the user. These devices can be used in a variety of electronic displays such as laptop computer displays.
In field emission displays it is desirable to align the extractor or grid to the emitter. The extractor, situated between the emitter and the screen, is charged such that it can extract electrons from the emitter and accelerate them toward the screen.
One technique for self-aligning the extractor to the emitter is to use a chemical mechanical polishing process. After the emitters are formed they may be covered with a generally conformal layer of oxide followed by a generally conformal layer of a conductive material such as silicon. Because of the conical shape of the emitter, the portion of the dielectric and conductive layers over the emitter, forms a hump or hillock on the semiconductor surface. By using a chemical mechanical polishing process the hillock can be removed down to the oxide layer, leaving in effect, an opening in the conductive layer which is self-aligned to the emitter. The conductive layer opening is self-aligned to the emitter because it was the shape of the emitter itself, in cooperation with the chemical mechanical polishing process, which defined the opening in the conductive layer. The conductive layer ultimately becomes the extractor or grid associated with the emitter after a portion of the oxide layer between the extractor and the emitter has been removed.
An emitter, formed over a junction in a semiconductor layer, can be controlled to emit electrons through its tip. These electrons pass from the emitter tip through the opening in the extractor and are accelerated by the extractor potential towards a screen. When the electrons hit the screen they cause luminescence which the user perceives as an image.
Sometimes field emission displays suffer from a leakage problem which causes lighter regions to appear on the screen. A leakage problem may arise from light that enters the junction under the emitter in one of several ways. Light can enter this region by being reflected back from the screen towards the underlying semiconductor layer. Also light from a variety of sources outside the display may enter the junction through the opening in the extractor and by actually passing through the extractor itself.
As a result of photoelectric affects, electrons may be created in the junction which are emitted through the emitter, operating the emitter even when the emitter is effectively turned off. Similarly, leakage from the junction to the underlying substrate may adversely affect the operation of the display.
The active matrix driving scheme of field emission displays requires integration of silicon devices and tips on a single silicon substrate. Such a scheme can be easily implemented by using MOS devices as shown in FIG. 1. Although this is a very efficient way of manufacturing small area field emission displays, it also suffers from serious drawbacks.
The main problem with the scheme illustrated in FIG. 1 is the fact that photons from the phosphor anode can easily pass through the extraction grid and consequently generate electron hole pairs in the MOS devices. Generated electrons will be attracted by the higher positive field in the tip area and cause a bright background. This problem is even more severe for color displays where three colors, red, blue, and green are used as the main ingredients of phosphor anodes. If the background light is not extremely dim, it can turn the wrong color on, cause cross talking among different colors, and distort the image quality of a display.
Light sensitivity is basically originated from the presence of the pn junction right under the tip, where generated electron hole pairs can be separated and find their paths to the tip area and substrate, respectively. This disclosure describes a new technique which can significantly reduce the light leakage problem.